(a) Field of the Invention
The present invention relates to a method for preparing cerium carbonate. More specifically, the present invention relates to a method for preparing cerium carbonate that can prepare cerium carbonate having uniform crystal structure, shape and size with high productivity.
(b) Description of the Related Art
Cerium oxide powder is a high functional ceramic material widely used as a catalyst, a phosphor, cosmetics, an abrasive, etc., and it has been recently spotlighted as an abrasive for a CMP (Chemical Mechanical Polishing) process in the field of semiconductor device.
The cerium oxide powder is commonly prepared by firing a precursor such as cerium carbonate at high temperature, and the physical properties or shape of the cerium oxide powder may be varied according to the crystal structure, shape or size (diameter) of the cerium carbonate. Therefore, to prepare cerium oxide powder having desired physical properties and shape, it is very important to uniformly obtain cerium carbonate having a controlled crystal structure, shape or size.
The cerium carbonate is generally prepared by introducing reactants such as a cerium precursor and urea in a batch type reactor of a certain size in a solution state and reacting them at an elevated temperature, and as the result, cerium carbonate in the form of powder is obtained. However, in this method, since heat transfer speed may become non-uniform at each part of the reactor if the size of the reactor exceeds a certain level, the crystal structure, shape or size of the prepared cerium carbonate powder may become non-uniform.
The reason therefor may be predicted as follows.
In the mechanism for the production of cerium carbonate powder particles, cerium carbonate that is initially produced is dissolved in a solvent until unsaturated, and the initial particles of cerium carbonate powder are formed after passing the unsaturation state. At this time, the initial particles should be rapidly and uniformly formed so that these initial particles equally grow in the solvent to prepare cerium carbonate having uniform size (diameter) and shape.
For the rapid and uniform formation of the initial particles, the decomposition speed of urea in the reaction solution is important. This is because urea is decomposed by heat to form ammonia and carbon dioxide, which react with a cerium precursor to form cerium carbonate. Specifically, if the decomposition speed of urea is slow or non-uniform, the initial particles of cerium carbonate may be non-uniformly formed and the degree of growth of each initial particle may differ, and thus, the shape or size of cerium carbonate may become non-uniform. Namely, the initial particles that are produced earlier have prolonged growth process and thus are prepared to cerium carbonate having larger diameter, while the initial particles that are produced later are prepared to cerium carbonate having small diameter, and thus, the shape or size of the finally prepared cerium carbonate may become non-uniform.
For this reason, to obtain cerium carbonate having more uniform shape and size and the like, heat transfer speed should be uniformized throughout the reactor to make the decomposition of urea uniform and rapid, however, in the existing preparation method of cerium carbonate using a batch type reactor, uniform heat transfer is difficult.
Meanwhile, it is known that the crystal structure of cerium carbonate may be varied according to reaction conditions such as reaction temperature and the like. Cerium carbonate may be divided into orthorhombic and hexagonal cerium carbonate according to the crystal structure, and it is commonly known that the orthorhombic cerium carbonate is prepared by a precipitation reaction at comparatively low temperature, and the hexagonal cerium carbonate is prepared by a high temperature high pressure reaction. However, in the existing method using a batch type reactor, since heat transfer may become non-uniform at each part of the reactor to vary reaction conditions including reaction temperature and the like, the crystal structure of the finally prepared cerium carbonate may become non-uniform if the size of the reactor exceeds a certain level.
Due to the above-explained problems, in the prior art, in case the size of the reactor was large beyond a certain level, the crystal structure, shape or size of the finally prepared cerium carbonate often became non-uniform, and thus, cerium oxide powder prepared from the cerium carbonate often had non-uniform physical properties, shape and size. Therefore, there was a limit to increase the size of the reactor, which had a bad influence on mass production of cerium carbonate.
To solve these problems, there has been an attempt to secure temperature uniformity inside the reactor by separately designing the shape or size of a stirrer, however, there is a limit to secure temperature uniformity inside the reactor simply by changing the size etc. of the stirrer as the size of the reactor increases.